1. Field
The present invention relates to a lead-free solder, and more particularly, to a lead-free solder alloy suited for micro-solder bonding for use in electronic components and to solder bonding using the solder alloy.
2. Background Art
In recent years, the use of lead-free solders is widespread in consideration of global environment. For example, Sn—Cu, Sn—Cu—Ni, Sn—Ag, Sn—Ag—Cu, Sn—Zn, Sn—Ag—Bi, Sn—Sb, Sn—Bi, Sn—In, Al—Si and Bi—Zn lead-free solder alloys and lead-free solder alloys obtained by appropriately adding Ni, Co, Ge, Ga, Cr, P, Si, Ti, V, Mn, Fe, Zr, Nb, Mo, Pd, Te, Pt, Au, etc. to the above-mentioned lead-free solder alloys have been proposed and are put into practical use. Of the above-mentioned lead-free solder alloys, in particular, Sn—Cu, Sn—Ag—Cu and Sn—Cu—Ni eutectic solder alloys are used frequently.
On the other hand, as personal computers, mobile terminals, etc. are downsized, electronic components to be used in those electronic devices are miniaturized. Also in the mounting of such electronic components, there is a strong tendency to micronization. Furthermore, in the mode of mounting electronic components, BGA (ball grid array) and CSP (chip size package) technologies and solder paste containing fine lead-free solder alloy powder are being used
Moreover, as a lead-free solder alloy suited for micronization in the mounting of electronic components, for example, a lead-free solder alloy having a composition containing 3.0 wt % of Ag, 0.5 wt % of Cu and the balance being Sn are put into practical use. However, since the solder alloy contains 3.0 wt % of Ag and is expensive, lead-free solder alloys being less expensive and having excellent bonding properties and high reliability are demanded, and various lead-free solder alloys are being developed and examined.
For example, Japanese Patent Application Laid-Open No. 2002-239780 exemplifies a lead-free solder alloy having a basic composition of Sn—Ag—Cu containing 1.0 to 2.0 wt % of Ag, 0.3 to 1.5 wt % of Cu and the balance being Sn, to which Sb, Zn, Ni and Fe are added appropriately, whereby the lead-free solder alloy is less expensive than the lead-free solder alloy containing 3.0 wt % of Ag and is superior in bonding reliability and drop impact resistance. However, the lead-free solder alloy has problems left unsolved in wettability and oxidation resistance at solder bonding portions, etc. in comparison with the conventional lead-free solder alloy having a composition containing 3.0 wt % of Ag, 0.5 wt % of Cu and the balance being Sn.
Furthermore, WO 2007/081006 exemplifies a lead-free solder alloy having a composition of Sn—Ag—Cu—Ni containing 0.1 to 1.5 wt % of Ag, 0.5 to 0.75 wt % of Cu, Ni satisfying the relationship of 12.5≦Cu/Ni≦100, and the balance being Sn.
However, as in the case of Japanese Patent Application Laid-Open No. 2002-239780, the lead-free solder alloy also has problems left unsolved in wettability and oxidation resistance at solder bonding portions in comparison with the conventional lead-free solder alloy having a composition containing 3.0 wt % of Ag, 0.5 wt % of Cu and the balance being Sn.
Moreover, Japanese Patent Application Laid-Open No. 2001-58286 exemplifies a solder paste consisting of a lead-free solder alloy having a basic composition of Sn—Ag containing 0.2 to 1.0 wt % of Ag and the balance being Sn, to which 1.0 wt % or less in total of Cu and/or Sb, 0.3 wt % or less in total of one or two or more selected from the group consisting of Ni, Co, Fe, Mn, Cr and Mo, 0.5 or more to 3.0 wt % or less in total of one or two or more selected from the group consisting of Bi, In and Zn, and 0.2 wt % or less in total of one or two or more selected from the group consisting of P, Ga and Ge are respectively added. However, the mode according to the patent disclosed in Patent Document 3 is limited to a paste, and the use thereof is also limited to the prevention of chip rising. Hence, versatile lead-free solder alloys, other than solder pastes, having compositions applicable to BGA, for example, are demanded.
What's more, the inventors of the present invention has proposed a lead-free solder alloy having a composition of Sn—Cu—Ni—Ga containing 2 wt % or less of Cu, 0.002 to 0.2 wt % of Ni, 0.001 wt % or less of Ga and the balance being Sn as disclosed in Japanese Patent Application Laid-Open No. 2008-142721. However, lead-free solder alloys having higher properties in wettability and drop impact resistance are demanded.